Place pumpkin cubes into a steamer in a saucepan. Add water, cover and bring to a boil over medium-high heat. Allow to steam until the pumpkin is tender (10 to 15 minutes) then drain and mash in a bowl with a fork.
Melt butter in a saucepan over medium-high heat. Add onion; cook and stir for 2 minutes until the onion begins to soften, then stir in the rice. Continue cooking and stirring until the rice is glossy from the butter and the onion begins to brown on the edges, about 5 minutes more.
Pour in the white wine; cook, stirring constantly, until it has evaporated. Stir in the mashed pumpkin and 1/3 of the hot chicken stock; reduce heat to medium. Cook and stir until the chicken stock has been absorbed by the rice, 5 to 7 minutes. Add half of the remaining chicken stock and continue stirring until it has been absorbed. Finally, pour in the remaining stock and continue stirring until the risotto is creamy. Finish by stirring in the Parmesan cheese and seasoning to taste with salt and pepper.